Long story short on tenacity: Tanks used to have parry as a substat on a gear, but it was avoided at all costs because it the vast majority of cases it was a dps loss or nearly insignificant dps gain, and the parry chance wasn't worth stacking parry for given its already rng nature. It would lead to higher ilvl weapons with Parry being inferior to lower ilvl weapons without it (I did the math back in HW, it was depressing).

Tenacity replaced Parry back in Stormblood, and although it isn't as good dps wise as determination, at least it doesn't result in a dead stat for dps and at least does something, which is more than parry ever did